At this point, we actually know a fair amount about how certain behaviors, attitudes, and choices relate to happiness, though most research on the topic can only find correlations.

Researchers think that roughly 40% of our happiness is under our own control; the rest is determined by genetics and external factors. That means there's a lot we can do to control our own happiness.

Here are nine happiness-promoting behaviors backed by science.

1. Relationships are essential. A major study followed hundreds of men for more than 70 years, and found the happiest (and healthiest) were those who cultivated strong relationships with people they trusted to support them.

2. Time beats money. A number of studies have shown that happier people prefer to have more time in their lives than more money. Even trying to approach life from that mindset seems to make people more content.

3. But it helps to have enough money to pay the bills. People's well-being rises along with income levels up to an annual salary of about $75,000, studies have found. (That number probably varies depending on your cost of living, however.)